Dissimilar air combat training (DACT) was introduced as a formal part of US air combat training after disappointing aerial combat exchange rates in the Vietnam War. Traditionally, pilots would undertake air combat training against similar aircraft. For example, pilots of F-8 Crusaders would seldom train against F-4 Phantom IIs, and almost never against A-4 Skyhawks and never as part of a formal syllabus. From 1965 to 1968, US pilots found themselves over the skies of North Vietnam pitted against the smaller, more nimble subsonic Soviet Mikoyan-Gurevich MiG-17 and the supersonic Mikoyan-Gurevich MiG-21. US pilots in USAF Republic F-105 Thunderchiefs were barely able to exceed parity and pilots in Phantoms and Crusaders were not able to achieve the hugely lopsided win–loss ratio achieved over Korea and in World War II. In fact, Air Combat Maneuvering (ACM) was not practiced by all fighter squadrons for a variety of reasons. The USAF had deemphasized ACM since most air combat doctrine since the late 1950s centered on delivering nuclear weapons over Europe or firing missiles at beyond-visual-range (BVR) at bombers, not daylight dogfighting which was thought to be obsolete in the missile age. The primary US fighter used against North Vietnamese MiGs, the F-4 Phantom, did not even have an internal gun. US pilots were finding themselves hard-pressed to prevail over the nimble VPAF MiGs which by late 1966 had grown to be a real threat to US aircraft operating over the North. Even more vexing were rules of engagement (ROE) that did not even permit BVR firing of missiles. Radar-guided AIM-7 Sparrows experienced high failure rates, and the short-range Sidewinder was ineffective in many dogfighting maneuvering situations. Phantom training against other Phantoms did not reflect the reality of a target that was smaller, smokeless and more agile. Ever since the success of the American Volunteer Group (Flying Tigers), aerial tacticians have advocated exploiting differences in aircraft to maximize one's own advantages while minimizing the disadvantages of one's own platform, thus neutralizing the superior maneuverability and climbing speed of, for example, a Zero compared to the rugged, fast-diving and powerfully armed P-40 Tomahawk. US pilots found themselves the victims of VPAF MiG-21s using Chennault's Flying Tigers "hit and run" tactics against them. The U.S. Air Force began to reinstate DACT in 1966 in Air Defense Command. Its Convair F-106 Delta Dart interceptor squadrons had been tasked with a worldwide mission to send expeditionary forces